A laser-homodyne diffusion study of agglutination of Streptococcus mutans and Streptococcus sanguis is to be carried out to elucidate the early stages of plaque formation. Methods have been devised which permit measurements of aggregation rates of plaque bacteri starting as early as fifteen seconds after mixing with submaxillary and/or parotid saliva and yielding for arbitrarily long time periods continuous data of aggregate sizes and shapes. Preliminary results on the effect of different environments (saline supensions with and without saliva) have been obtained and are to be extended to other chemical and biological systems. Comparisons between caries-resiitant and caries-susceptible patients are to be made. The technique consists of preparing small in vitro samples of bacteria mixed with the chemical factors under study, saliva, etc. The sample is illuminated with laser radiation, and the Doppler shifted light scattered from the bacteria is collected and analyzed electronically to give the homodyne power spectrum. The diffusion data contains the size and shape of the aggregates. A simple computer program is used to determine mean aggregate size as a function of time. The procedure, though novel, may be suitable for clinical applications. Interbacterial effects will also be studied using several species of oral bacteria and selectively dyeing one of the species so that it could be studied apart from the others. Furthermore, chemical factors found in the oral cavity can be studied for their inductive, inhibiting or dispersive effects on bacterial aggregates. The simplicity and accuracy of the optical techniques suggests their clinical applicability for the screening of patients and subsequnt treatment with anti-plaque and anti-bacterial agents.